1. Field of the Invention
The present invention relates generally to a method of controlling a magnetization easy axis in ferromagnetic films using voltage, an ultrahigh-density, low power, nonvolatile magnetic memory using the control method, and a method of writing information on the magnetic memory.
More particularly, the present invention relates to a method of controlling the spin directions of ferromagnetic films using an inverse magnetostrictive effect by applying voltage to a piezoelectric film instead of applying a magnetic field and so exerting tensile or compressive stress on magnetic films, and a method of writing information on a Magnetic Random Access Memory (MRAM) using voltage to put the nonvolatile, high density, low power magnetic memory to practical use.
2. Description of the Prior Art
Recently, a key issue in the field of information storage technology is the implementation of an ideal nonvolatile information storage device. Promising candidates for the implementation of such an ideal nonvolatile information storage device are a Ferroelectric Random Access Memory (FeRAM) using the high-charge spontaneous dipole phenomenon of a ferroelectric film and a Magnetic Random Access Memory (MRAM) using a spin polarization phenomenon. The MRAM is a nonvolatile magnetic memory device that can compare with the FeRAM having both the advantage of rapid speed of a Static Random Access Memory (SRAM) and the advantage of high density of a Dynamic Random Access Memory (DRAM).
In spite of having the great advantage of non-volatility, the MRAM has a difficulty in realizing ultrahigh integration in that it is difficult to localize an external magnetic field with its sufficient strength when spacing between cells is reduced and so the deletion of information may be caused by interaction between the spins of cells. In magnetization direction control by applying a magnetic field, the localization of the magnetic field becomes difficult in inverse proportion to the size of the cell when the spin direction of each cell is switched to store information. That is, when a conventional spin switching driving method using a magnetic field is employed, there occurs a problem in writing information on an ultrahigh density integrated memory in which the size of cells and spacing between the cells are reduced. This is because the applied magnetic field affects neighboring cells that do not need to be spin switched, so recorded bits may be deleted. Additionally, stored spin directions, that is, information, may be deleted by interaction between the spins of cells. Accordingly, for the ultrahigh density MRAM, it is essential to control spontaneous magnetization directions without applying a magnetic field through current along a metal wire, and it is necessary to eliminate an interference effect caused by interaction between recorded spin directions. Further, in the conventional MRAM technology, the relative spin directions of fixed and free magnetic films are read by using the magnetoresistance effect of tunneling electrons passing through an insulating film layer that separates the two ferromagnetic films from each other, so the thickness of the insulating film must be less than about 1 nm. It is difficult to deposit the insulating film having a regular thickness of 1 nm onto a wafer having a radius of several inches, so it is a significant shortcoming of the conventional MRAM device.
Recently, challenging attempts have been made to control magnetization directions by applying current instead of using an applied magnetic field according to a conventional method. One of these attempts is the prediction of spin switching in a complicated layered structure of ferromagnetic/metallic spacer/insulator/ferromagnetic, which is possibly caused by controllable exchange coupling, but is not experimentally proven yet (refer to You, C. Y. and Bader S. D., Prediction of switching rotation of the magnetization direction with applied voltage in a controllable interlayer exchange coupled system, J. Magn. Magn. Mater. 195, 488-500 (1999)). Another attempt is the experimental demonstration of current-induced switching of spin directions in Co/Cu/Co sandwich structures, and such phenomenon is ascribed to local exchange interactions between flowing conduction electrons and magnetization (refer to Myers, E. B., Ralph, D. C., Katine, J. A., Louie, R. N. and Buhrman, R. A., Current-Induced Switching of Domains in Magnetic Multilayer Devices, Science 285, 867-870 (1999)). However, the latter case has a problem in that current to induce spin switching is usually used for measuring Giant Magnetoresistance (GMR) as well.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method of controlling spontaneous magnetization directions in ferromagnetic films operated at room temperature by using voltage, and more particularly a voltage-driven spin switching method in a hybrid system of piezoelectric/magnetic films using inverse magnetostrictive and inverse piezoelectric effects, and a method of writing information on a nonvolatile magnetic memory, such as a MRAM, in which 128 or more Mbit ultrahigh integration is implemented.
Another object of the present invention is to provide a magnetic memory that is advantageous in that an insulating thin film having a thickness of about 1 nm is not employed, thus eliminating the difficulty of manufacturing and improving the yield of manufacturing.
A further object of the present invention is to provide a method of writing information on a magnetic memory, which uses a magnetization easy axis and so vertical and horizontal relative spin directions instead of using opposite spin directions on a single magnetization easy axis, so stable information storage function is provided, thus overcoming the deletion of information caused by a superparamagnetism effect that occurs when the size of cells is reduced.
In order to accomplish the above object, the present invention provides a method of controlling a magnetization easy axis of a ferromagnetic film, comprising the steps of arranging an electrode layer, a piezoelectric layer and a magnetic layer in a layered structure; applying voltage to the electrode layer to generate an electric field, wherein the electric field causes a lattice change (expansion or compression) in the piezoelectric layer, and stress (tensile stress or compressive stress) onto the magnetic layer induced by the lattice change yields a switching of the magnetization easy axis between in-plane and out-of-plane in a reversible way.
In accordance with another feature of the present invention, the piezoelectric layer is comprised of any one selected from the group consisting of PZT, PLZT, BLT and SBT which is tolerant of fatigue.
In accordance with another feature of the present invention, the piezoelectric layer has a thickness equal to or less than 100 nm, and charge dipole polarization is generated at less than several volts when voltage is vertically applied, so a nonvolatile, low power memory device on which information can be written with low voltage can be manufactured. In the case where the piezoelectric layer has a thickness equal to or less than 50 nm, a very low power memory device having a drive voltage of less than 1 volt can be manufactured.
In accordance with an additional feature of the present invention, a magnetic element constituting the magnetic layer is comprised of a CoPd alloy. Alternately, the magnetic element may be comprised of a CoFe and NiFe alloy, or ternary alloy comprised of Ni, Fe and Co to achieve great electrostriction and magnetoresistance characteristics. Also, a non-magnetic element among Pd, Pt, Au, Cu, Ru, W may be added to the alloy.
In accordance with an additional feature of the present invention, the electrode layer comprises metallic electrode lines made of any one selected from the group consisting of Pt, Pd, Cu, Al, Ru and W.
In accordance with an additional feature of the present invention, a nonvolatile, ultrahigh density, low power magnetic memory using a method of controlling the spin directions of a ferromagnetic film comprises a memory cell array, which includes a piezoelectric layer comprised of a piezoelectric element; a free magnetic layer disposed on the piezoelectric layer, wherein a magnetization easy axis is reversibly switched between in-plane and out-of-plane by stress induced by the piezoelectric layer applied voltage; a fixed magnetic layer which is disposed on the free magnetic layer and whose magnetization axis is fixed; and a nonmagnetic layer interposed between the free magnetic layer and the fixed magnetic layer to suppress magnetic interaction between the two magnetic layers.
In accordance with an additional feature of the memory device of the present invention, the memory cells are connected to one another by perpendicular metallic electrode lines, and the magnetization easy axis of the free magnetic layer is switched by applying voltage to the metallic lines connected to the memory cells when information is written in memory cells.
In accordance with an additional feature of the memory device of the present invention, information is read out by reading the relative spin directions of the free and fixed magnetic layers in such a way as to apply current to the metallic electrode lines and read the magnetoresistance value of the free and fixed magnetic layers, that is, a magnetoresistance value between vertically and horizontally arranged spins.
In accordance with an additional feature of the memory device of the present invention, the spin directions are determined by vertical and horizontal magnetization easy axes, so the spin directions are free from interaction between the spins of the cells.
In accordance with an additional feature of the memory device of the present invention, the spin directions of the spin-fixed layer and the spin-free layer are determined by magnetization easy axes, so the magnetization easy axes can be maintained under a superparamagnetism effect, which is generated when the size of the cells is reduced, by strong magnetic anisotropy caused by stress, and the deletion of information can be suppressed.
In accordance with an additional feature of the memory device of the present invention, when voltage is applied in a vertical direction of the memory cell plane, diagonal metallic electrode lines are additionally arranged, and information is read out by applying current to the diagonal and horizontal metallic electrode lines connected the memory cells. In this case, 3 metallic lines are necessary for writing and reading information.
In accordance with an additional feature of the magnetic memory of the present invention, each of the metallic electrode lines is divided into a path for applying voltage and a path for applying voltage by an insulator so as to prevent the flow of current through the magnetic layers when the voltage is applied in a horizontal direction of the memory cell plane.